1. Field of the Invention
This invention relates to the field of comparators, and particularly to comparator-based circuits for selecting the most positive of several available input voltages.
2. Description of the Related Art
For some electronics applications, multiple power forms are available to power the circuitry. For example, a voltage regulator's input voltage might be selected from any of several available primary and auxiliary supply voltages.
In some applications of this sort, it is useful to determine which of the available voltages is the highest. For example, in a linear voltage regulator, the control circuits which operate the pass device should be referred to a voltage at least as high as the input voltage selected for regulation. In addition, for a regulator implemented with field-effect transistors (FETs), the n-well of any FET connected to the regulated output should connect to the most positive voltage available, to avoid forward biasing its body diode(s).
One or more comparators might typically be used to determine the highest available voltage. However, a comparator as might be used for this purpose must itself be powered from one or more supply voltages; these supply voltages establish the comparator's input voltage range. However, this range might be exceeded by one or more of the voltages being compared, which could render the comparator's output unreliable. The comparator's output voltage swing would also be defined by its supply voltages. As such, the output might not swing between the largest available voltages, and thus might be unsuitable for use by circuits intended to be responsive to the output.